title: Evidence for a close spatial location of the binding sites for CO2 and for photosystem II inhibitors.
abstract: 1. CO2-depletion of thylakoid membranes results in a decrease of binding affinity of the Photosystem II (PS II) inhibitor atrazine. The inhibitory efficiency of atrazine, expressed as I50-concentration (50% inhibition) of 2,6-dichlorophenolindophenol reduction, is the same in CO2-depleted as well as in control thylakoids. This shows that CO2-depletion results in a complete inactivation of a part of the total number of electron transport chains. 2. A major site of action of CO2, which had previously been located between the two electron acceptor quinone molecule B (or R) and Photosystem II inhibitor atrazine as suggested by the following observations: (a) CO2-depletion results in a shift of the binding constant (kappa b) of [14C]atrazine to thylakoid membranes indicating a decreased affinity of atrazine to membrane; (b) trypsin treatment, which is known to modify the Photosystem II complex at the level of B, strongly diminishes CO2 stimulation of electron transport reactions in CO2-depleted membranes; and (c) thylakoids from atrazine-resistant plants, which contain a Photosystem II complex modified at the inhibitor binding site, show an altered CO2-stimulation of electron flow. 3. CO2-depletion does not produce structural changes in enzyme complexes involved in Photosystem II function of thylakoid membranes, as shown by freeze-fracture studies using electron microscopy.
mesh_terms: D001280:Atrazine / Q000494:pharmacology*; D001639:Bicarbonates / Q000494:pharmacology; D002245:Carbon Dioxide / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism*; D004579:Electron Transport; D005614:Freeze Fracturing; D007425:Intracellular Membranes / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D007700:Kinetics; D008854:Microscopy, Electron; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis / Q000187:drug effects*; D010944:Plants / Q000378:metabolism; D014357:Trypsin / Q000494:pharmacology
chemical_list: D001639:Bicarbonates; D002245:Carbon Dioxide; D014357:Trypsin; D001280:Atrazine; D010100:Oxygen